Electromechanical microwave switches use electromagnetic actuators to change switch states by moving switch active elements such as RF reeds. Electro-magnetic switch actuators need to provide latching to allow the microwave switch to be powered up for only a short time period during switching. Intrinsic latching maintains the switch state during mechanical vibrations or shocks, ensures good electrical contact between the contacts, and provides extra reliability. Electromagnetic switch actuators also need to have low mass and small volume since actuators typically account for more than one half of the switch mass. The inertia forces are proportional to the mass of the mobile armature, and therefore the amount of latching force/torque necessary to maintain the switch position increases with mass, requiring a higher active force and larger actuator.
Electromechanical switches employed in microwave communications are generally either switches with rotary actuators or switches with linear actuators. Linear electromagnetic actuators basically break down into three categories, namely electromagnetic actuators (that utilize the tractive force), voice coil actuators (that utilize the Lorentz force), and solenoid actuators (that utilize the reluctance force). There are several weaknesses associated with each of these types of linear actuators. Electromagnetic actuators, voice coil actuators and solenoid actuators do not have an intrinsic latching mechanism and accordingly an external separate latching mechanism is generally required. For electromagnetic actuators and solenoid actuators, since actuation is only possible in a single direction, the use of either elastic elements (e.g. springs) or additional actuators are required to provide bi-directional functionality. Further, linear actuators generally exert their lowest force at the beginning of the stroke and their highest force at the end of the stroke. This is problematic since a large force is required at the beginning of the stroke in order to overcome latching forces. If actuators are simply made larger to overcome latching forces, the increased (i.e. very high) force at the end of the stroke results in excessively high mechanical impacts on switch contacts. Finally, voice coil actuators having a size that is compatible with microwave switch applications do not generally provide sufficient magnetic force for practical microwave switch applications.
More specifically, as shown in FIG. 1, electromagnetic actuators utilize an electromagnet 2 having stationary coils which attract a mobile armature 5. The tractive force F that is associated with the electromagnet 2 is related to the magnetic flux Φ that exists within the air-gap of the electromagnet 2, the magnetic permeability of free space μ0, the area of pole regions A, the magnetomotive force of the coil mmf, the number of turns of the electromagnetic coil N, the electric current I through the electromagnet 2, the magnetic reluctance Rmk for the circuit element k, the length Lmk of the circuit element k and the equivalent magnetic reluctance Rme of the circuit. The direction of the tractive force F generated does not depend on the direction of the current due to the fact the value of magnetic flux is squared in the force relation. Accordingly, a switch actuator that utilizes tractive force F is not bi-directional. Also, the magnetic force is minimal at the maximum gap since the magnetic reluctance is highest at the maximum gap resulting in lowest flux value. Conventional switch tractive force based actuators utilize armatures made of soft magnetic material that provide no intrinsic latching and must rely on external elements to provide latching. The tractive force based actuator disclosed in U.S. Pat. No. 5,075,656 to Sun et al. utilizes an armature made out of a permanent magnet to achieve intrinsic latching and bi-directional motion. However, changing the armature from soft magnetic material to a permanent magnet results in a significant increase in the reluctance of the magnetic armature since μPMAGNET<<μSOFT CORE. Accordingly, the magnetic flux and the magnetic force will decrease significantly. For these reasons, these types of actuators are of very limited use and can be used only where an exceptionally short stroke is adequate.
FIG. 2 illustrates the basic operating principle of the Lorentz force upon which voice coil actuators are based. The interaction of a magnetic field B with the current I in a coil wire 3 generates the well-known Lorentz force. Either the coil wire 3 or the armature can be used as the mobile element within the actuator. The formulas listed in FIG. 2 that are used to calculate force F are based on the assumption that a charge q is traveling a length L of coil wire 3. The direction of the magnetic force generated depends on the direction of the electric current I running through a coil wire 3. Accordingly, the actuator is bi-directional. There is no intrinsic latching associated with a voice coil actuator based only on the Lorentz force since the force results only from interaction between the current I and the magnetic field B. For a constant current I, the force magnitude F is quasi-constant with the stroke. This is due to the fact that the force magnitude F depends only on magnetic flux density. The flux density remains constant because the magnetic flux direction is perpendicular to the direction of the stroke. The major disadvantage of a conventional voice coil actuator for microwave switch applications is that increasing the number of coil turns does not increase the magnetic force F generated. Rather, increasing number of turns increases the gap which in turn results in a decrease of the magnetic flux that intersects the coil turns. A voice coil actuator having a size and mass that is compatible with typical microwave switch dimensions can only generate a maximum force in the vicinity of 10 grams, which is not sufficient in practice for microwave switch applications.
Conventional solenoid actuators are normally constructed by winding a coil of wire 6 around a moveable soft iron core plunger 4 as shown in FIG. 3. Wire coil 6 is wound around plunger 4 and current is provided to the coil in such a direction such that the portion labeled as “A” represents current flowing out of the plane of the figure and that the portion labeled as “B” represents current flowing into the plane of the figure. Accordingly, the direction of the magnetic flux Φ is shown by the arrowed line surrounding coil 6. As shown, reluctance force F is exerted upon plunger 4. The direction of the reluctance force F does not depend on the direction of the current since as with tractive force based actuators, the value of magnetic flux is squared in the force relation as shown. Accordingly, the solenoid actuator is not bi-directional. The direction of the force depends only of the direction that reduces the reluctance. The force is minimal at the maximum gap. Conventional solenoid actuators utilize soft magnetic material and as such possess no intrinsic latching. In an attempt to obtain bi-directional motion, solenoid actuators have been designed to utilize a permanent magnet for the plunger 4 as disclosed in U.S. Pat. Application No. 2002/0,008,601 to Yajima et al. However, in such a case, the reluctance of the plunger will increase significantly since μPMAGNET<<μSOFT CORE and the magnetic flux and the magnetic force will decrease causing the actuator to be inefficient. Another variant of the conventional solenoid actuator is the use of an additional elastic element (e.g. springs) to achieve the return stroke as disclosed U.S. Pat. No. 6,133,812 to Magda or U.S. Pat. No. 5,724,014 to Leikus et al. However, it is not desirable because the mechanical characteristics of elastic elements (e.g. springs) vary during the course of the actuator life and as such, important switch parameters, such as contact forces, latching stiffness etc. vary over time.